The pitch control system of a conventional wind turbine generally includes an emergency pitch control capability to protect the wind turbine from an overspeed condition in the event of loss of AC control power or initiation of an emergency shutdown. Upon detection of a dip or loss of AC power (e.g., from a loss of grid power), the system activates to pitch the blades to a position to slow or stop the hub. Known emergency pitch control systems utilize a backup battery bank of, for example, 4 to 8 batteries assigned to each blade pitch drive for this purpose.
U.S. Pat. No. 7,740,448 describes a battery backup pitch control system wherein energy stored in a DC link capacitor is first used to operate the pitch control system in the event of a loss or dip in AC input power. A charged backup battery maintains charge on the DC link as voltage drops during operation of the emergency system. A diode is used in the circuit to prevent uncontrolled charging of the backup battery when DC link voltage is higher than battery voltage. A fuse prevents damage to the battery in the event of a short circuit on the DC link.
U.S. Pat. No. 7,642,748 describes a system for charging a string of backup batteries connected in series in the emergency pitch control system of a wind turbine. A battery charger is coupled in parallel to each respective battery for independent and charge profiles customized to specific battery requirements.
For systems wherein multiple chargers are assigned to respective batteries within a ban, such as the system in the U.S. Pat. No. 7,642,748 discussed above, certain battery failure modes may result in damage to the chargers. One such mode is a reverse polarity condition that occurs when the battery fails in an open circuit or high impedance failure mode and an emergency pitch operation is performed. In this instance, the voltage applied on the bad battery and its respective charger will be reversed, with this reversed polarity voltage being the sum of the remaining batteries in the bank. The other mode occurs when a battery in the bank opens transiently when the bank is connected to the pitch drive motor. The energy stored in the motor windings will cause a large transient current that will damage the charger.
The present invention provides a system to protect the battery chargers from the failure modes discussed above.